Gasoline typically includes a mixture of hydrocarbons ranging from higher volatility butanes (C4) to lower volatility hydrocarbons (C8 to C10). When vapor pressure increases in the fuel tank due to conditions such as higher ambient temperature or displacement of vapor during filling of the tank, fuel vapor flows through openings in the fuel tank. To prevent fuel vapor loss into the atmosphere, the fuel tank is vented into a canister (known as an evaporative canister or adsorbent canister) that contains an adsorbent material such as activated carbon granules or pellets. The evaporative canister is part of a system directed to controlling the emission of fuel vapors generated by fuel carried in the vehicle's fuel system. These evaporative emission control systems (“EVAP” systems) are implemented as a collateral system to the fuel system.
When the gasoline tank is filled, fuel vapor accumulates in the canister. The fuel vapor is a mixture of gasoline vapor (referred to in this description also by its main component, hydrocarbon vapor) and air. The initial loading may be at the inlet end of the canister, but over time the fuel vapor is gradually distributed along the entire bed of the adsorbent material. As the fuel vapor enters an inlet of the canister, the hydrocarbon vapor is adsorbed onto activated carbon granules and the air escapes into the atmosphere. The size of the canister and the volume of the adsorbent activated carbon are selected to accommodate the expected gasoline vapor generation.
After the engine is started, the control system uses engine intake vacuum to draw air through the adsorbent to desorb the fuel. A purge valve between the vehicle's engine and the EVAP system opens and air is drawn through the canister. The air removes fuel vapor that is stored in the adsorbent material. The desorbed fuel vapor is directed into an air induction system of the engine as a secondary air/fuel mixture to be consumed in the normal combustion process.
In a hybrid vehicle including both an internal combustion (IC) engine and an electric motor, the IC engine is turned off nearly half of the time during vehicle operation. Because the purging takes place only during operation of the IC engine when the desorbed vapor can be consumed in engine combustion, in a hybrid vehicle the adsorbent canister purging with fresh air occurs less than half of the time. A hybrid vehicle generates nearly the same amount of evaporative fuel vapor as does a conventional vehicle having an IC engine. Therefore, the lower purge rate of the hybrid vehicle is not sufficient to clean the adsorbed fuel out of the adsorbent canister.